Japan rejected Chinese protests Monday over the raising of a Japanese flag on disputed islands but sounded a placatory note, saying ties with Beijing are among the "most important" it has.

Tokyo stood firm in its insistence that islands where Japanese nationalists landed on Sunday, which it administers, were part of its territory, but said it wanted to improve ties with its giant neighbour.

The comments came as Chinese media rounded on Japan after street protests erupted across China over a series of moves that Beijing considers provocative.

Japan's Chief Cabinet Secretary Osamu Fujimura said Beijing and Taipei, which both claim the islands, had lodged objections after 10 Japanese nationalists landed on what Japan calls the Senkaku islands and China calls Diaoyu.

Sunday's flag-raising came just days after Tokyo deported pro-Beijing protesters who had landed on the same island.

"We have explained our nation's basic position and told them that we cannot accept their claims," Fujimura told a news conference in Tokyo.

"There is no doubt that the islands are our sovereign territory historically and under international law, and our nation controls the islands," he added.

Despite their large and mutually important trade relationship, ties between Tokyo and Beijing are often blighted by historical animosities, especially war-time atrocities carried out by the invading Japanese army.

But Fujimura insisted that neither Tokyo nor Beijing had any interest in seeing overall relations affected by the dispute over the islands, whose seabed is believed to harbour rich mineral resources.

"The Japan-China relationship is one of the most important bilateral ties for Japan," he said.

"China's constructive role is necessary for the stability and prosperity of the Asia-Pacific region," he said. "We would like to continue to further progress mutually beneficial relations between Japan and China."

Sentiment in the Chinese media was more strident, after thousands of people in more than 20 cities protested on Sunday, in what some analysts said was the biggest wave of anti-Japanese sentiment since 2005, when several cities were rocked by violent demonstrations.

"Japan is building another wall in its relations with China and the Japanese intruders and their government seem hell-bent on freezing Sino-Japanese ties," the English-language China Daily said in an editorial.

"It would be a mistake for Japan to see China's use of reason and restraint to deal with the Diaoyu Islands dispute as its weakness," it added.

The People's Daily newspaper, the mouthpiece of China's ruling Communist party, said Japan should recognise the consequences of its actions, which damaged Sino-Japanese relations.

The Global Times newspaper, known for its nationalistic stance, warned China could reciprocate if Japan increased its defence of the islands.

"China will definitely take further steps regarding Diaoyu," it said. "The reluctance to resort to military means doesn't mean China is afraid of war."

Fujimura called on the Chinese government to ensure the safety of Japanese nationals in China after Japanese businesses, restaurants and cars were targeted.

The Japanese foreign ministry has separately issued a travel advisory, telling its nationals to be on alert while staying in the country.

In far southern Okinawa, police were questioning the 10 Japanese nationalists who had landed on the island without the necessary permission.

A police spokesman told AFP the questioning was being done on a voluntary basis as officers tried to "get a handle on what happened".

The police were examining whether the landing was "appropriate under the law", he said while declining to give further details.

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